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•  The adhesive property of water and evaporation generate tension forces in leaf cell walls

Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata

  • Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
  • This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
  • This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
  • The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls

Evaporation within Leaf Tissue


Regulating Water Loss

The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata

  • Guard cells flank the stomata and can occlude the opening by becoming increasingly flaccid in response to cellular signals
  • When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
  • Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
  • A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening

Transpiration rates will be higher when stomatal pores are open than when they are closed

  • Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration
  • Other factors that will affect transpiration rates include humidity, temperature, light intensity and wind

Closing of Stomata by Abscisic Acid

guard cells